1. Field of the Invention
The present invention relates to a stepping motor control method, and more specifically, to a stepping motor control method of securely moving a head to a predetermined track position of a disc-shaped recording medium such as a floppy disc or the like by a stepping motor.
2. Description of the Related Art
Conventionally, a recording/reproducing apparatus making use of a disc-shaped recording medium such as a floppy disc or the like has used a head driving mechanism including a stepping motor in order to move a head, which records and reads data to and from the disc-shaped recording medium, to a predetermined track position of the recording medium.
In this case, the head driving mechanism is composed of a carriage for holding the head, a motor driving circuit and a pulse generation circuit in addition to the stepping motor and the pulse generation circuit is connected to a controller composed of a microcomputer and the like disposed externally of the recording/reproducing apparatus. The stepping motor has a rotor directly coupled with an output shaft which is screwed to the carriage. When the output shaft is rotated simultaneously with the rotation of the rotor of the stepping motor, the carriage screwed to the output shaft is moved linearly, whereby the head which is held by the carriage is also linearly moved on the disc-shaped recording medium in the diameter direction thereof. Thus, the head moves from one track position to a predetermined track position.
In the above arrangement, when the stepping motor is driven and the head is moved from the one track position to a predetermined track position of the disc-shaped recording medium, the stepping motor is driven under the control executed in the following sequence. First, the controller generates a step pulse and a direction designation pulse and supplies them to the pulse generation circuit. Next, the pulse generation circuit generates a pulse for switching the excitation phase of the stepping motor in response to the thus supplied step pulse and direction designation pulse and supplies the pulse to the motor driving circuit. Subsequently, the motor driving circuit amplifies the pulse having been supplied thereto and then applies the pulse to the excitation phases of the stepping motor as a driving current and rotates the rotor and the output shaft of the stepping motor forwardly and reversely in accordance with the exciting polarity of the driving current. At the time, the carriage screwed to the output shaft is linearly moved on the disc-shaped recording medium in the inside diameter direction or the outside diameter direction thereof by the forward or reverse rotation of the output shaft to thereby move the head to the predetermined track position. When the head reaches the predetermined track position, the motor driving circuit stops the supply of the driving current to the excitation phases of the stepping motor and stops the head at the predetermined track position to thereby permit the head to access the track.
Incidentally, it is known that the stepping motor for moving the head ordinarily has a rotation characteristics that it rotates and stops at a different position when a load is imposed on the rotor and the output shaft, that is, so-called hysteresis rotation characteristics.
FIG. 5 is a characteristic graph showing the output torque characteristics of the stepping motor, wherein the ordinate represents torque T and the abscissa represents rotational angle .theta. of rotor.
As shown in FIG. 5, the stepping motor is arranged such that when the load imposed on the output shaft (rotor) is in an ideal no load state, a rotational force is applied to the rotor until an output torque becomes 0even if the rotor is rotated in any one of a forwardly direction and a reverse direction and when the rotor is stopped at a rotational angle .theta..sub.0 at which the output torque is made to 0. On the other hand, when any load is imposed on the output shaft (rotor) and the output torque T is within the range of a minute torque (.+-.Tf) on both the sides of the center where a torque is zero, any rotational force is no longer applied to the rotor and the rotor stops at the time. In this case, since the rotational angle .theta. at which the rotor stops is determined by the magnitude of load imposed on the output shaft, the inertial of the load and the like, the position where the rotor stops corresponds to any rotational angle in the range from the rotational angle -.theta.f to the rotational angle +.theta.f. When the rotor rotates forwardly, the hysteresis rotation characteristics are exhibited at a position which corresponds to a rotational angle near to the rotational angel -.theta.f, whereas when the rotor rotates reversely, the hysteresis rotation characteristics are exhibited at a position which corresponds to a rotational angle near to the rotational angel +.theta.f. Thus, the stepping motor has a dead zone within the range of the rotational angel (.+-..theta.f).
Since the dead zone ordinarily has a width of about 10 .mu.m when it is converted into the moving distance of the head, the width is smaller than the width of the track of the disc-shaped recording medium such as the floppy disc or the like which is about 110 .mu.m.
As described above, the hysteresis rotation characteristics make it difficult to properly stop the head at a predetermined position on a predetermined track, preferably at a track center in the recording/reproducing apparatus which moves the head using the stepping motor. Further, when an error resulting from the assembling accuracy of the head and a temperature environment and the like in which the apparatus is used is added to the hysteresis rotation characteristics, the head may be moved to a track another than a predetermined track and may not be set to the predetermined track.
To prevent the above disadvantage, there has been proposed a head moving means for eliminating a head stop position error which is caused by the hysteresis rotation characteristics of a stepping motor in Japanese Unexamined Patent Publication No. 60-210198.
The head moving means according to Japanese Unexamined Patent Publication No. 60-210198 is arranged such that after a head is moved to a predetermined track position by a stepping motor, the stepping motor is repeatedly driven in a forward direction and a reverse direction by the switching of the excitation phases thereof and after it is rotated in any one of the directions, the rotation of the stepping motor is stopped and the head is properly stopped at a predetermined position on a predetermined truck, that is, at a track center.
The means according to Japanese Unexamined Patent Publication No. 60-210198 eliminates the head stop position error caused by the hysteresis rotation characteristics of a stepping motor and stops a head at the track center of a predetermined track position though not quite satisfactorily. Since the head is positioned in the dead zone of the stepping motor, however, the stepping motor must be repeatedly rotated in the forward direction and the reverse direction many times by the switching of the excitation phases thereof to correctly stop the head at the track center. Thus, the means has a problem that many operations are required to position the head.
An object of the present invention for solving the above problem is to provide a stepping motor control method capable of positioning a head in the dead zone of a stepping motor relatively simply as well as correctly.